Sensor Calibration

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Sensor Calibration

• Mark Seyfried Soil Scientist
• Agriculture Research Service (ARS) Boise, ID

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Background

• The most direct and accurate measurement of soil
  water content is by gravimetric sampling, which
  involves physically collecting a sample of
  soil, weighing it, oven drying it at 105 C, and
  then weighing it again. This has some serious
  limitations---
• Its destructive
• You have to account for spatial variability of
  soil properties and wetting front dynamics
• You have to go and visit the site to get a
  measurement
• You cant collected high resolution data (either
  in time or vertically in the soil profile.
• For most hydrology purposes you also need to know
  the bulk density.

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Background

• With the Hydra Probe, or any automated sensor,
  you do not measure soil water content directly.
  You measure some electrical property of the soil
  that is affected by soil water.
• For many years the resistance of the soil was
  used (Coleman Blocks, Gypsum blocks, Watermark).
  That is, the electrical resistance of the soil
  was related to the water content.
• These have never received widespread use because
  they are very temperature sensitive and because
  the relationship between resistance and water
  content varies considerably among different
  soils.

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New Sensors

• Newer (since 1980s) sensors are based on
  measuring the dielectric permittivitywhich is
  related to the capacitance
• The dielectric permittivity of water is about 80,
  while that of mineral soil is about 5 and air
  about 1. Thus, the water content pretty much
  determines the bulk or mixed soil dielectric
  permittivity.
• Various formulations have been worked out to make
  a universal soil calibration that relates the
  dielectric permittivity to soil water content for
  all soils.
• In the 1990s a number of new sensors developed
  and marketed using this principal, including the
  Hydra Probe.
• But, life aint that simple.

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Why Calibration

• Commercial sensors work at different frequencies
  than the experimental ones
• The universal calibration only works for very
  sandy, non saline soils. Soil clays have
  electrical properties that alter the dielectric
  permittivity so that the universal calibration
  does not work.
• We tested the applicability of the universal
  calibration to three soils from Reynolds Creek,
  Idaho, and construction sand using an
  experimental sensor (TDR or time domain
  reflectometry), CS615 (commercial sensor) and the
  Hydra Probe. You will see that it works well with
  TDR and horribly with the CS615. All sensors work
  great in sand which has no clay to mess things
  up.
• The Hydra Probe is much better than the CS615,
  but there are substantial differences among
  soils.
• Thats why we are concerned about the calibration.

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Control cell
Plug
Hydra-Probe
Stand
Tempe cell
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• Experimental datathe range of 19 soils
• Mean or general calibration equation along with
  the loss corrected result
• Topp is close to the mean, quite different from
  manufacturer supplied calibrations

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• Side-by-side comparison of TDR and HP with
  general calibration.
• Basically gives the same answer in light clay
  loam soil.
• Note diurnal fluctuations during snowmelt.

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• Typical progression
• Freezing effect, temperature accuracy
• Snowmelt timing

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Reynolds Mountain East
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